Recently, people have proposed various measures for reducing the viewing angle dependence of the γ characteristic of a liquid crystal display panel. The “γ characteristic” is a grayscale luminance characteristic. If the γ characteristic has viewing angle dependence, the γ characteristic when the panel is viewed straight on and the γ characteristic when the panel is viewed obliquely will be different from each other, and therefore, the grayscale display state will also vary according to the viewing direction (i.e., the viewing angle).
To overcome this problem, according to a known method (see Patent Document No. 1, for example), each pixel is divided into multiple subpixels and mutually different effective voltages are applied to respective liquid crystal layers of those multiple subpixels. Such a method of dividing a single pixel into multiple subpixels is generally called a “pixel division” and various circuit structures have been developed for that purpose.
Meanwhile, Patent Document No. 2 discloses a liquid crystal display panel in which multiple regions with mutually different pretilt angles are defined in the liquid crystal layer of a pixel by using the polymer sustained alignment technology (which will be referred to herein as a “PSA technology”). According to the PSA technology, a small amount of a polymerizable material (such as a photo-polymerizable monomer) is introduced into a liquid crystal material, and the polymerizable material is irradiated with an active energy ray (which is typically an ultraviolet ray and is supposed to be an ultraviolet ray in the following description) with a predetermined voltage applied to the liquid crystal layer after the liquid crystal cell has been assembled, thereby controlling the pretilt azimuth and pretilt angle of liquid crystal molecules with the polymer thus produced. In the following description, to control the pretilt azimuth and pretilt angle of liquid crystal molecules will be sometimes referred to herein as “giving a pretilt to the liquid crystal molecules”. The alignment state of the liquid crystal molecules when the polymer is produced is sustained (i.e., memorized) even after the voltage is removed (i.e., no longer applied). In this description, the pretilt azimuth is defined by the azimuth angle within a substrate plane (e.g., when the display screen is compared to a clock face, the three o'clock direction may have an azimuth angle of zero degrees and the angle may increase counterclockwise) and the pretilt angle is defined to be the angle formed with respect to the substrate plane (which corresponds to the angle of elevation, and the pretilt angle along a normal to the substrate plane is 90 degrees). The pretilt angle in each region of the liquid crystal layer becomes the average of the pretilt angles of liquid crystal molecules which are located close to the surface of an alignment film in that region and the pretilt angles of liquid crystal molecules which are located everywhere else. In the following description, the pretilt angle to be given to those liquid crystal molecules which are located close to the surface of an alignment film will be sometimes referred to herein as the pretilt angle of that alignment film (or alignment region).
As methods for forming multiple regions with mutually different pretilt angles in each pixel, Patent Document No. 2 discloses the following two methods (1) and (2):
(1) By introducing a structure for applying mutually different voltages to the liquid crystal layer of multiple regions in a pixel, the polymerizable material included in the liquid crystal layer may be polymerized with mutually different voltages applied to those multiple regions in each pixel, thereby making the pretilt angles different from one region to another. In this case, every region of a single pixel is irradiated at the same time with an ultraviolet ray for the purpose of polymerization.
(2) Or the process step of irradiating only a particular region of a single pixel which has been selected through a photomask with an ultraviolet ray and producing a polymer in only that particular region of the pixel may be performed a number of times. In this case, when the regions to irradiate an ultraviolet ray with are changed, the voltages applied to the liquid crystal layer are also changed. According to this method (2), such a structure for applying mutually different voltages to the liquid crystal layer of multiple regions in a pixel is not needed.
Recently, as methods for giving a pretilt to liquid crystal molecules (i.e., as methods for controlling the pretilt azimuth and pretilt angle), not only the PSA technology but also methods disclosed in Patent Documents Nos. 3 to 5 are known as well. According to the methods disclosed in Patent Documents No. 3 to 5, no polymerizable material is added to the liquid crystal material and no unreacted polymerizable material will be left in the liquid crystal layer of the liquid crystal display panel, unlike a situation where the PSA technology is adopted.
Patent Documents Nos. 3 to 5 disclose a method for giving a pretilt to liquid crystal molecules using an alignment film material including a polymer material with a cross-linkable site (i.e., a cross-linkable group). Specifically, with a voltage applied to a liquid crystal layer that is interposed between two substrates, each of which is coated with such an alignment film material including a polymer material with a cross-linkable site, the alignment film material is irradiated with an ultraviolet ray, thereby causing the cross-linkable site to cross-link and forming an alignment film with a cross-linking structure. This alignment film with such a cross-linking structure functions to fix the alignment state of liquid crystal molecules which are located close to the alignment film material when the material is irradiated with the ultraviolet ray. That is why even after the voltage that has been applied to the liquid crystal layer is removed, the liquid crystal molecules that are located close to the alignment film will sustain the alignment state that has been regulated by the electric field. In this manner, the alignment film gives a pretilt to the liquid crystal molecules.
Patent Document No. 6 discloses a liquid crystal display device which divides a counter electrode that faces pixel electrodes into multiple sub-counter electrodes and which applies mutually different effective voltages to respective liquid crystal layers of multiple pixels, thereby minimizing a decrease in the aperture ratio of the display area and suppressing the whitening phenomenon efficiently. The entire disclosure of Patent Document No. 6 is hereby incorporated by reference.